While programming a nonvolatile semiconductor memory cell array, e.g., a stacked-gate memory cell in which each memory cell has a floating gate and a control gate, in order to “inject” electrons onto the floating gate, accelerated electrons traveling in a depletion region must collide with impurities or lattice imperfections in the substrate to generate momentum in a direction toward the floating gate. Further, only those electrons having sufficient velocity in the direction of the floating gate to overcome the energy barrier at the silicon-oxide interface (i.e., substrate-gate oxide interface) plus the potential change across the floating gate oxide will be injected onto the floating gate. As a result, only a small percentage of electrons (e.g., on the order of one in one million) from the programming current in the depletion region will have sufficient energy to be injected onto the floating gate.
In addition, programming electrons often experience an electric field in the depletion region that is unfavorable in the direction of the floating gate. The electric field accelerates the electrons in various directions away from the floating gate. As a result, only a small percentage of electrons from the programming current will have sufficient energy to overcome the unfavorable electric field and be injected onto the floating gate.